Medical diagnostic imaging employing silver halide emulsions for X-radiation imaging can be divided into two categories: (1) direct X-ray imaging and (2) indirect X-ray imaging.
In the first approach a radiographic element is constructed with a silver halide emulsion layer coated on a support. The ability of the silver halide grains to absorb X-radiation during exposure is relied upon for imaging. When a single emulsion layer is employed for X-radiation capture, only about 1 percent of the exposing radiation is absorbed. The absorption can be doubled by coating two emulsion layers, one on each major face of a film support. Such elements are term "dual-coated" and are sold by the Eastman Kodak Company under the trademark "Duplitized".
In the second approach an intensifying screen is used in conjunction with a silver halide emulsion layer. The intensifying screen captures X-radiation and emits light that is used to imagewise expose the emulsion layer. The ability of the emulsion to absorb light is about 20 times greater than its ability to absorb X-radiation. Hence patient dosage is markedly reduced. Dual-coated radiographic elements intended for indirect exposure minimize patient exposure to X-radiation and serve the majority of medical diagnostic applications.
One of the problems encountered with dual-coated indirect X-ray films occurs when light emitted by an intensifying screen on one side of a transparent film support inadvertently crosses over to expose the emulsion layer on the opposite side of the support. The problem of crossover results in reduced image sharpness. This problem can be eliminated by adding to dual-coated radiographic elements crossover control layers between the emulsion layers and the support. Reduced crossover dual-coated indirect X-ray films are disclosed by Dickerson et al U.S. Pat. Nos. 4,803,150, 4,900,652, 4,994,355, 4,997,750, 5,021,327, 5,108,881 and 5,399,470.
Dental diagnostic imaging conventionally employs dual-coated direct X-ray films. Nothing akin to the crossover problem observed in indirect X-ray films is observed. During X-ray exposure no light is generated. Due to its high energy level, the X-radiation is either absorbed or passes straight through the element and hence the type of image unsharpness generated by light crossover is entirely avoided. Zietlow U.S. Pat. No. 5,370,977 illustrates a modern direct X-ray film construction.
In dental diagnostic imaging the area of a single exposure is small. A small piece of X-ray film, commonly referred to as a "chip", mounted in a sealed opaque package, is placed in the patient's mouth during X-ray exposure. The oral location of the film during exposure renders the use of intensifying screens difficult and, when the small area of exposure and therefore small benefit to be gained is taken into account, impractical. Following exposure, the dentist or dental technician retreats to a separate room equipped with safelights to remove the dental film chip from its opaque package and complete processing to a viewable image.
Attempts to integrate light exposure protection into direct X-ray films are known, but have not been accepted. For example, Murray U.S. Pat. No. 2,379,373 discloses overcoating the emulsion layers of direct X-ray films with carbon black in a casein and gelatin vehicle that can be removed during processing. Boucher U.S. Pat. No. 2,542,304 discloses laminating strippable opaque layers over the surface of the emulsion layers in direct X-ray films.